A microlithography projection exposure apparatus for producing microelectronic components and a related method are known, for example, from U.S. Pat. No. 6,295,119B1 and U.S. Pat. No. 6,526,118B2.
Microlithography projection exposure apparatuses used for producing microelectronic components include, among other things, a light source and an illumination system for illuminating a structure-bearing mask (the so-called reticle), and a projection optical unit for imaging the mask onto a substrate (the wafer). The substrate contains a photosensitive layer that is altered chemically upon exposure. This is also referred to as a lithographic step. In this case, the reticle is arranged in the object plane and the wafer is arranged in the image plane of the projection optical unit of the microlithography projection exposure apparatus. The exposure of the photosensitive layer and further chemical processes give rise to a microelectronic component.
Microlithography projection exposure apparatuses are often operated as so-called scanners. This means that the reticle is moved along a scanning direction through a slotted illumination field, while the wafer is moved correspondingly in the image plane of the projection optical unit. The ratio of the speeds of reticle and wafer corresponds to the magnification of the projection optical unit, which is usually less than 1.
In this case, the optical components of the illumination system and of the projection optical unit can be either refractive or reflective or diffractive components. Combinations of refractive, reflective and diffractive components are also possible. The reticle can likewise be embodied either in reflective fashion or in transmissive fashion. Such apparatuses are composed completely of reflective components particularly when they are operated with a radiation having a wavelength of less than approximately 100 nm, in particular of between 5 nm and 15 nm.
Such a microlithography projection exposure apparatus can have a limited illumination field and also a limited field that can be imaged. It may be desirable, however, to image a structure-bearing mask into the image plane, in which the substrate with the photosensitive layer is arranged, even if the mask is so large that either it cannot be completely imaged or it cannot be completely illuminated.
If the mask is larger only in one direction than the region that can be illuminated or imaged, the microlithography projection exposure apparatus can be operated as a scanner, such that the mask is moved through the slotted illumination field in the direction, while the wafer is moved correspondingly in the image plane of the projection optical unit. This means that, at least in principle, a mask of arbitrary size in the direction can be illuminated and imaged.
However, if the mask is larger in both directions than the region that can be imaged or illuminated, it may not be possible to rectify this scanning. In such a case, the structure-bearing mask is divided into at least two partial regions that are individually imaged or illuminated. This can conditionally be combined with a scanning process. In this case, the midpoints of the at least two partial regions are at a distance perpendicularly to the scanning direction, such that the combination of the at least two partial regions is larger than each of the individual partial regions. In combination with the movement in the scanning direction, a relatively large structure-bearing mask can thus be illuminated and imaged.
In order, however, to give rise overall to a complete image of the mask structure in the photosensitive layer, it is helpful if the partial regions at least partly overlap. This makes it possible to ensure that there are no regions of the mask which are inadvertently not imaged or not illuminated. However, these overlap regions cause problems in the configuration of the structure-bearing mask. Particularly in the cases where the mask is not illuminated perpendicularly, in the production of the mask it is desirable to take account of which centroid direction of the radiation is present at a point of the mask in the projection exposure apparatus. These effects can become worse the more the centroid direction of the radiation differs from perpendicular illumination. One may consider these problems if an angle between the first centroid direction and a normalized vector that is perpendicular to the mask at each point of at least one partial region of the overlap region is 3° or larger, especially 6° or larger.
The centroid direction of the incident radiation is understood to mean the average direction of the incident radiation. If a point is illuminated uniformly from all directions of a beam cone, then the axis of symmetry of the beam cone coincides with the centroid direction. In the case of non-uniform illumination, generally an energy-weighted average is formed, in which each direction is weighted with the intensity of the radiation coming from this direction. The centroid direction is then the average energy-weighted direction.
The centroid direction is taken into account in the production of the mask since shadow casting and projection effects that distort the image of the mask can occur during oblique illumination. Shadow effects can occur because such a structure-bearing mask is not completely planar. In the case of a reflective mask, the non-reflective regions are raised since at these locations one or more covering layers have been applied to one or more reflective base layers. Such a three-dimensional construction of the mask can therefore lead to shadow effects.
Shadow and projection effects can be taken into account in the production of a mask, however, with the result that the desired image arises in the image plane of the microlithography projection exposure apparatus.
If the overlap region is illuminated and imaged twice, this can give rise to specific desired properties for the centroid directions of the radiation of first and second exposures, in order still to be able to take account of shadow casting and projection effects.